Prevalence of Disorders in the Autism Spectrum

This study of autism and other disorders in the autism spectrum over time aimed to ascertain whether there has been an increase in the prevalence of these disorders in recent years. All English language papers on the prevalence of autism were reviewed, and 20 saidies, published in 1966 through 1997, were located and subdivided into U.S. studies (n=2) and non-U.S. studies (n=18). The latter group was subdivided into four 8-year periods. The studies relating to other disorders on the autism spectrum using acceptable methodology were few and far between. The early studies yielded autism prevalence rates of under 0.5 in 1,000 children, whereas the later showed a mean rate of about 1 in 1,000. There was a marked difference in prevalence rates between those studies that included some children born before 1970 (low) and those that included only children born in 1970 and after (high rates). This could be due to lower rates obtained by use of criteria strictly based on Kanner's description of his syndrome. The U.S. studies reported atypically low rates. There was a highly significant estimated change with calendar year in the non-U.S. studies (3.8% per year). Asperger syndrome appears to be much more common than autistic disorder, whereas the syndrome described by Heller (disintegrative disorder) is much rarer. The prevalence of other autism spectrum disorders remains uncertain. Autism is considerably more common than previously believed. There are several possible reasons for the higher rates, including change in diagnostic criteria and increasing autism awareness in society generally. Asperger syndrome is possibly much more common than autism, and the population rate of autism spectrum disorders (including the syndromes delineated by Kanner, Asperger, and Heller) appears to be at least half a per cent of the general population of school age children.